An early, common type of automotive air conditioning condenser was the so called serpentine condenser, in which one refrigerant flow tube (or sometimes one tube pair) tube was continually folded back and forth on itself in a meandering pattern. All refrigerant flowed through the single tube or tube pair, back and forth, from one end to the other. Despite an inherent efficiency limitation of a high refrigerant pressure drop resulting from the long flow path, the design was simple and robust. Only two potential leak paths, at the two ends of the single tube, had to be sealed, and very few parts were involved in its manufacture.
Since at least the early 1980's, there had been a natural progression in the automotive industry away from serpentine, single tube condensers to multi flow tube condensers, most accurately referred to as headered or cross flow condensers. Headered condensers include a pair of opposed, parallel, elongated manifolds or header tanks, which distribute refrigerant into and out of a plurality of much shorter flow tubes, each about as long as one bend in an equivalent serpentine design. The header tanks, in turn, have a single discrete refrigerant inlet and outlet that feed and drain them of refrigerant. The header tanks are generally vertical (so that the flow tubes are horizontal), although that pattern may be reversed in a so-called down flow design. Since each single flow tube is much shorter than the single tube of equivalent capacity serpentine design, the pressure drop across each individual tube is far less. The smaller potential pressure drop, in turn, allows smaller flow passages within each flow tube, which inherently increases heat transfer efficiency. The main drawback of the headered design is that each of the two ends of each shorter flow tube must be sealed where they enter the header tanks, which greatly multiplies the potential leak points. Improvements in the brazing process widely available in the late 70's and early 80's have essentially obviated that concern, however, and accelerated the shift toward the headered design.
One inherent drawback of the headered condenser design, however, is the inability of the header tanks to feed refrigerant into and out of the individual flow tubes evenly. This is exacerbated when the tanks are long and the number of flow tubes large, inevitably putting the ends of many of the tubes far distant from the single, discrete header tank inlet and/or outlet, especially the inlet. The problem is even worse when the inlet is near the upper end of a vertical tank, as it often is. Tubes closer to the discrete inlet will have a surplus refrigerant flow, those more distant a flow deficit. This is a problem that has been long recognized, but the proposed solutions to date have been impractical from a manufacturing standpoint.
One potential solution would be to create an inlet header tank which, rather than having a uniform cross sectional area along its length, is larger at points more distant from the inlet, so as to feed more refrigerant to the tubes that would otherwise be starved of flow. However, condenser tubes are most often made of an aluminum extrusion, which has to have a uniform cross section along its length. The obvious equivalent of a varying cross section tank would, instead, be flow tubes with a varying flow passage size, those more distant from the inlet being larger and vice versa. An equivalent to varying flow passage size tubes would be the use of tube end blocking structures that effectively blocked part of the otherwise fully open ends of those flow tubes nearer the inlet, leaving the more distant tubes more open or fully open. Making and accounting for different thickness flow tubes would be impractical and expensive, as would adding individual tube end blocking structures, however, and the extra cost would not be worth the efficiency gain.
An early reference that extolled the benefits of shifting from a serpentine to a headered condenser design also recognized the inherent problem of refrigerant flow imbalance. Laid Open Japanese Utility Model 57-66389, published in 1982, proposed a couple of solutions, one of which is impractical in some cases, and the other of which is impractical in all cases. The sometimes practical approach is the well known process of "multipassing" the flow. Baffles or separators, which are internal dams that completely block flow at selected points along the length of the header tanks, cause the flow to run back and forth in a large scale imitation of serpentine flow. One baffle yields two passes, two yield three, and so on, although it would be rare to provide more than three passes. Since each flow pass has fewer than all tubes in it, fewer tubes are as distant from the inlet or outlet, and the flow is more even through those passes. The pressure drop is greater than for a single pass design with no baffles, but efficiency can be increased in many cases, and a sufficient increase in efficiency is worth a tolerable pressure drop increase. The totally impractical approach proposed is to feed a fraction of the total refrigerant flow directly into each flow tube separately with dedicated, capillary pipes, one for each end of each flow tube. These individual tube feeders radiate out like tines of a fork from a central distributor, and occupy a great deal of space on the sides of the core. With anything more than a handful of flow tubes, such an approach would be impossible from a manufacturing and packaging standpoint.
Even the theoretically practical approach of multi passing is unusable in many cases, again because of packaging concerns. Often, the lines to the refrigerant inlet and outlet must be located on opposite sides of the condenser. This is fine for a single pass condenser, since the inlet and the outlet (on opposite tanks) are already on opposite sides of the condenser. But a two-pass design, with its U shaped flow pattern, puts the inlet and outlet on the same header tank and same side of the core. A long cross over pipe would be necessary to connect the outlet back to the opposite side of the condenser. A three pass design, with a "Z" shaped flow pattern, would put the inlet and outlet back on opposite sides, but the pressure drop will often be too great with three passes, and the outlet will be forced to the bottom lower corner, which may be an inconvenient location for it.
Therefore, a single pass condenser design is often the only practical design for many vehicle architectures. When a large plurality of flow tubes is used with a single pass design and vertical header tanks, yet another problem can present itself, in addition to the inevitable flow imbalance described above. Often, the inlet or outlet or both will be located high up on the vertical tanks, again, because of vehicle architecture and packaging constraints. This creates the potential for liquid refrigerant to pool in the lower flow tubes, which are the tubes most distant from the inlet and outlet, under the force of gravity. The pooled liquid refrigerant further blocks refrigerant vapor flow through the very flow tubes, the lower tubes, that already have a deficit of refrigerant vapor flow, and forces it up and through the upper tubes that have a surplus of flow. The effective working area of the condenser is greatly reduced. This liquid pooling/gas blockage problem is not an issue with heat exchangers that comprise all liquid flow, like radiators and heater cores, so radiator and heater core design features related to fluid flow are not useful per se in solving the pooling problem.